Ratio-dependent quantity discrimination in quorum sensing ants

To optimise behaviour, organisms require information on the quantity of various components of their environment, and the ability of animals to discriminate quantity has been a subject of considerable recent interest. This body of research hints at generalised mechanisms of quantity discrimination in vertebrates, but data on invertebrates are still relatively scarce. In this study, I present data on the quantification abilities of an invertebrate in a novel context: quorum sensing. Quorum sensing generates a behavioural response in group-living animals once a threshold number of individuals, a ‘quorum’, is detected performing some key action. This process forms the basis for consensus decision-making in many species and allows group-living organisms to decide among mutually exclusive alternatives without compromising group integrity. To determine when a quorum is achieved, individuals must assess the number of group members performing the key action. Social insects employ quorum decisions to decide among potential nest sites when searching for a new home. In the Japanese ant, Myrmecina nipponica, quorum thresholds increase with colony size, providing an opportunity to assess the accuracy of quantity discrimination at different stimulus magnitudes. In this study, I demonstrate that the variation in individual quorum thresholds around the mean increases with increasing colony size. This indicates that the quantity discrimination ability of ants decreases with stimulus magnitude, and thus exhibits ratio dependence in the manner of Weber’s Law. This may have implications for the accuracy of consensus decision-making and other collective actions in a range of group-living organisms.